1. Field of the Invention
The present invention relates to a stratified-scavenging two-stroke internal combustion engine.
2. Related Background Art
The so-called piston-ported two-stroke internal combustion engine has an exhaust port to exhaust combustion gas from a cylinder chamber, inlet port to introduce an air-fuel mixture into a crank chamber, and scavenging passages communicating with the cylinder chamber and crank chamber. These exhaust and inlet ports and scavenging passages are closed and opened by a piston moving up and down.
In the two-stroke internal combustion engine of this type, the cylinder chamber is scavenged by introducing the air-fuel mixture from the crank chamber into the cylinder chamber through the scavenging passageways during the exhaust stroke. Thus, at the time of scavenging, the “blow-by of air-fuel mixture” phenomenon is liable to occur, in which the air-fuel mixture is exhausted to the outside of the engine through the exhaust port without contributing to the combustion. Because of this “blow-by” phenomenon, piston-ported two-stroke internal combustion engines used in many hand-held power working machines involve the problem that it is difficult to reduce harmful substances in their exhaust gas.
To prevent the “blow-by” phenomenon, stratified-scavenging two-stroke internal combustion engines have been proposed as in International Patent Publication WO 98/57053 (hereafter referred to as “Patent Document 1”) and U.S. Pat. No. 6,571,756 (hereafter referred to as “Patent Document 2”). Patent Document 1 proposes that the air-fuel mixture is to be introduced into the crank chamber while air for scavenging is introduced near the scavenging ports of the scavenging passageways to charge the latter with the air. According to the invention disclosed in Patent Document 1, when the piston forced to descend by combustion of fuel opens the exhaust port and starts the exhaust stroke, fuel-free air in the scavenging passageways is introduced from the scavenging passageways into the cylinder chamber, which is thus scavenged. Then the air-fuel mixture in the crank chamber is charged in the cylinder chamber through the scavenging passageways.
Patent Document 2 proposes that the air-fuel mixture is to be introduced into the crank chamber as in Patent Document 1 while scavenging (fuel-free) air is introduced into the cylinder chamber through a reed valve. Also, that document proposes that the air-feeding port formed in the cylinder wall to discharge air into the cylinder chamber through the reed valve is to be disposed on the side of the exhaust port while the air-fuel mixture port also formed in the cylinder wall to discharge the air-fuel mixture from the crank chamber into the cylinder chamber is to be disposed in a position away from the exhaust portion.
According to the invention disclosed in Patent Document 2, the piston forced to descend by combustion of fuel opens the exhaust port and starts the exhaust stroke, while fuel-free air is introduced from the air-feeding port into the cylinder chamber and the air-fuel mixture is introduced from the crank chamber into the cylinder chamber through the air-fuel mixture port. Since the air-feeding port is disposed nearer to the exhaust port than the air-fuel mixture port, the air flowing from the air-feeding port into the cylinder chamber forms an air layer between the combustion gas in the cylinder chamber and the air-fuel mixture flowing into the cylinder chamber from the air-fuel mixture port. The air layer prevents occurrence of “blow-by of the air-fuel mixture” during scavenging.
The aforementioned piston-ported two-stroke internal combustion engine is lightweight because it is simple in structure. This type of engine provides a relatively high output. Thus, the two-stroke internal combustion engine is used as a source of power in hand-held tools such as brush cutters, chain saws, etc. which should be lightweight and compact. On this account, more and more two-stroke internal combustion engines are made from an aluminum alloy. However, a piston made from a light metal such as aluminum alloy should appropriately be protected against overheating.
It is therefore desirable to overcome the above-mentioned drawbacks of the related art by providing a stratified-scavenging two-stroke internal combustion engine in which the “blow-by of the air-fuel mixture” can effectively be prevented and the piston can be cooled appropriately.
It is also desirable to provide a stratified-scavenging two-stroke internal combustion engine in which a necessary and sufficient amount of air can be used for scavenging the cylinder inside.